The present invention relates to the material of a novel lithium ion conductive glass electrolyte useful as the material of electrolyte for a lithium cell.
The utilization of a solid material of electrolyte instead of a liquid material of electrolyte for a cell serves to eliminate problems of a cell wherein use is made of a liquid material of electrolyte, such as damage due to liquid leakage and leaching of an electrode material, and further brings about many additional advantages including easy attainment of thin film formation and a reduction in the size.
Known such cells include a lithium cell wherein the material of a lithium ion conductive solid electrolyte is used and a lithium metal is used as a negative electrode active material.
The material of a lithium ion conductive solid electrolyte is broadly classified into an organic polymer, an inorganic crystal and an inorganic glass.
Among them, the organic polymer has excellent moldability and can easily be formed into a thin film but has drawbacks such as poor mechanical strengths and thermal stability.
Although the inorganic crystal has excellent thermal stability and can be used at a high temperature, the polycrystalline nature thereof brings about drawbacks such as a lowering in the conductivity due to IR drop in the grain boundary, poor moldability and difficulty of thin film formation.
On the other hand, the inorganic glass has advantages such as excellent moldability, possibility of thin film formation and freedom from IR drop in the grain boundary. Therefore, if a material having a high ionic conductivity could be found, there is a high possibility that the inorganic glass can be applied to a lithium cell. However, in order to attain a high conductivity, it is necessary to incorporate a large amount of lithium ions in the glass. An increase in the lithium ion content makes it difficult to be vitrified. This unfavorably makes it necessary to rapidly quench a melt by means of a twin roller for vitrification.
For this reason, no material of a lithium ion conductive solid electrolyte which can be put to practical use has hitherto been developed.
Accordingly, the present inventors have paid attention to an inorganic glass among the materials of a lithium ion conductive solid electrolyte and made studies with a view to solving the above-described problems while enjoying the above-described advantages and, as a result, have found that a composition comprising a lithium halide as a major component and, incorporated therein in the form of a halide, alkaline earth metal ions and alkali metal ions other than lithium each in a predetermined proportion can be vitrified without the necessity for remarkable quenching and the resultant glass has an excellent lithium ion conductivity, which has led to the completion of the present invention.